To reliably detect the cosmic microwave background (CMB) anisotropy is of
great importance in understanding the birth and evolution of the Universe. One
of the difficulties in CMB experiments is the domination of the measured CMB
anisotropy maps by the Doppler dipole moment from the motion of the antenna
relative to the CMB. For each measured temperature the expected dipole
component has to be calculated separately and then subtracted from the date. A
small error in dipole direction, antenna pointing direction, sidelobe pickup
contamination, and/or timing synchronism, can raise significant deviation in
the dipole cleaned CMB temperature. After a full-sky observation scan, the
accumulated deviations will be structured with a pattern closely correlated to
the observation pattern with artificial anisotropies on large scales, including
artificial quadrupole, octopole, etc in the final CMB map. Such scan-induced
anisotropies on large scales can be predicted by the true dipole moment and
observational scan scheme. Indeed, the expected scan-induced quadrupole pattern
of the WMAP mission is perfectly in agreement with the published WMAP
quadrupole. With the scan strategy of the Planck mission, we predict that
scan-induced anisotropies will also produce an artificially aligned quadrupole.
The scan-induced anisotropy is a common problem for all sweep missions and,
like the foreground emissions, has to be removed from observed maps. Without
removing the scan-induced effect, CMB maps from COBE, WMAP, and Planck as well,
are unreliable for studying the CMB anisotropy.

Here they derive a general way to express many possible observational effects (timing offsets as in their previous work, errors in determining the sidelobe response of the detector, etc) as an effective error in the antenna line of sight direction which is constant in the spacecraft coordinate frame. They also come up with a way to clean errors of this form from maps, and demonstrate that they can recover the true input quadrupole from a simulated map even with a large artificial quadrupole on top of it.

What I find especially interesting here is that they actually make some predictions for Planck, so this will help resolve the issue of what's actually going on. Although they are careful to say that they can't fully calculate what's expected from Planck before the Planck data is released: "Note that the exact templates of scan-induced anisotropies can be calculated when the real Planck data release is available. ... Our calculations are based on the simulated Planck scanning scheme with assumed initial conditions of scan geometry. The real pattern of the scan-induced Planck quadrupole could hence be some kind of rotation or sign reversion to the ones shown here." Regardless, should be very interesting to revisit when the first Planck cosmological results appear!